The invention relates to a pneumatically actuable disc brake having a caliper straddling a brake disc. Brake pads press against both sides of the brake disc. A brake-application-side brake pad is actuable using a rotary lever of a brake application unit, which lever is pivotable around an eccentric axis. A plunger of a brake cylinder, which is actuable using compressed air, engages on the rotary lever.
Such a pneumatically actuable disc brake is known, for example, from EP 0 553 105 B1 and EP 1 230 491 B1.
The rotary lever, which is formed on one end as an eccentric and is supported on the caliper, is produced very solidly as a steel forged part or from a high-strength cast iron material because of the high operating forces to be transmitted.
Because of the large lever transmission ratio, for example, i=L1: e=15.6:1 (L1=length of the lever arm, e=eccentricity), which results from the ratio between the brake force to be applied and the applied force of the brake cylinder, from which a correspondingly large pivot angle of the brake lever in turn results, correspondingly high friction losses of the eccentric mounting result. Roller bearing half-shells, which encompass the eccentric mounting, are used to reduce the friction losses.
The design of the lever is performed in such a manner that no bending torques and deformations in the caliper resulting therefrom are caused by the force transmission from the roller bearings to be supported to the eccentric bearing.
However, the production of such roller bearings is only possible with a substantial manufacturing expenditure, having correspondingly high production costs.
In addition, such a roller bearing is naturally relatively susceptible to malfunction, which hinders an optimized service life.
It is therefore proposed in cited EP 1 230 491 B1 that friction bearings, which are distinguished by particularly high robustness, in addition to low production costs, be used exclusively for mounting the brake lever on the caliper.
However, the coefficients of friction of the maintenance-free friction bearings which are used are multiple times greater than the rolling resistance value of the roller bearings, which naturally has an effect on the provision of a brake force to be applied by the brake cylinder.
Due to the relatively large bearing diameter, a distance results from the eccentric rotational axis to an engagement point on the brake-application-side brake pad, from which an additional bending torque acting on the brake lever results. In order to keep a deformation of the brake lever resulting therefrom small, it is implemented as very solid, although the mentioned deformations and the increased stroke requirement of the brake cylinder resulting therefrom cannot be entirely avoided.
Overall, the known disc brakes of the type described above are incapable of meeting the requirements placed in this regard, in particular with respect to the required installation space, the existing weight, and the efficiency.
The invention is based on the object of improving a disc brake of the type described above in such a manner that it is optimized with respect to its installation space, its weight, its efficiency, and its production costs.
This and other objects are achieved by a disc brake having a caliper straddling a brake disc. Brake pads press against both sides of the brake disc. A brake-application-side brake pad is actuable using a rotary lever of a brake application unit, which lever is pivotable around an eccentric axis. A plunger of a brake cylinder, which is actuable using compressed air, engages on the rotary lever. A self-energizing unit is provided, which has a self-energizing factor selected such that the brake automatically releases after braking events. The rotary lever is supported at least on a spherical bearing element, which is mounted in the caliper and forms the eccentric rotational axis. The lever presses against a bearing ball, which is situated in the direction of the plunger engagement above the bearing element, and is positioned in a brake ram connected to the brake-application-side brake pad.
Through the self-energizing device provided according to the invention, which heretofore has been used exclusively in electro-mechanically actuable disc brakes, the power demand of the brake cylinder is reduced, with the result of a substantial installation space reduction, while the operating behavior of the brake remains the same.
A substantial reduction of the overall size of the brake cylinder is already possible using relatively small self-energizing factors. The self-energizing is selected by appropriate dimensioning of wedge or ramp angles in such a manner that reliable automatic release of the brake still results even in the case of a maximum possible coefficient of friction of the brake pads.
In contrast to electro-motively actuated disc brakes, the force generator for applying the brake therefore does not also have to be used for release.
The overall space reduction is possible in particular through the smaller stroke, which is reduced from 65 mm to 24 mm, for example. Similarly, the required actuation energy is also reduced to one-third of the prior actuation energy. This allows significantly smaller dimensioning of the friction bearing elements of the brake lever, whereby the bending torque acting on the brake lever is also reduced.
In addition to this desired effect, which results through the invention, a significant reduction of the bearing friction is also reached, which is achieved, on the one hand, by the smaller dimensioning and, on the other hand, by the minimized required pivot angle of the brake lever, which results in an improvement of the mechanical efficiency.
The mentioned reduction of the pivot angle results from a decrease of the lever transmission ratio which, as noted at the beginning, is 15.6:1 in a disc brake according to the prior art, while it is in the range of from 7:1 to 10:1, preferably 7.5:1, i.e., noticeably smaller, due to the invention.
With the aid of the invention, an overall length reduction of the brake cylinder is therefore possible, as well as a weight savings, which corresponds to the requirements in this regard, from which a fuel savings is derived overall. This is also contributed to by the fact that the brake lever can be designed in a weight-optimized manner due to the reduced bending strain thereof, the brake lever only corresponding with one brake ram, which engages centrically in the axial plane in relation to the rotational axis of the brake disc, and which is supported by two pressure rams. The two pressure rams are situated on both sides of the brake ram and engage on one end side on the caliper and on the other end side on the brake-application-side brake pad. The pressure rams dissipate their reaction force directly into the caliper.
The brake lever is preferably produced as a formed part without machining and can be implemented as a relatively thin-walled forged part having embossed bearing seats, as a type of precision forged part, or as a sheet-metal formed part. Fundamentally, of course, manufacturing from a high-strength cast iron material using subsequent machining is also possible.
According to a further aspect of the invention, the brake lever is provided with an angled, at least partially peripheral edge, by which a high stiffness is achieved at low weight. In addition, stiffening beads or the like can be introduced.
To increase the strength, the brake lever insert can be hardened or quenched and tempered.
The bearing receptacles, in the form of spherical caps, are provided with a friction-reducing and/or wear-reducing coating, or may be lined using metal-composite friction bearing shells.
For the drive of a wear adjustment unit, a tooth contour which is capable of engaging in an adjuster drive wheel can be embossed on the brake lever or fastened on as a separate part by welding, for example.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.